Liposomes are widely described in the literature and their structure is well known. They are formed by amphipathic molecules such as the class II polar lipids, that is, phosphatidyl cholines, ethanolamines and serines, sphingomyelins, cardiolipins, plasmalogens, phosphatidic acids and cerebrosides. Liposomes are formed when phospholipids or other suitable amphipathic molecules are allowed to swell in water or aqueous solutions to form liquid crystals usually of multilayer structure comprised of many bilayers separated from each other by aqueous material. Another type of liposome is known consisting of a single bilayer encapsulating aqueous material which may also be referred to as a unilamellar vesicle. "If water-soluble materials are included in the aqueous phase during the swelling of the lipids they become entrapped between the lipid bilayers. Alternatively, lipid soluble materials may be dissolved in the lipid and, hence, may be incorporated into the lipid bilayers themselves," Ryman, B. E., "The Use of Liposomes as Carriers of Drugs and Other Cell-Modifying Molecules," Proc. 6th Int'l. Congr. Pharmacol. 5, 91 (1976), published in "Drug Applications," Clinical Pharmacology, vol. 5, pp. 91-103, Pergamon Press (1975).
In recent years there has been much interest in the use of liposomes as carriers of compounds which are of interest because of one or other biological property, for example, medicaments, proteins, enzymes, hormones and diagnostic agents, hereinafter referred to as "biologically active compounds." Liposomes have been suggested as carriers for drugs, see Ryman, supra at page 91 and Gregoriadis, G., "Enzyme or Drug Entrapment in Liposomes: Possible Biomedical Application," Insolubilized Enzymes, Ed. M. Salmona et al., Raven Press, N.T. 1974, pp. 165-177.
Water-soluble materials are encapsulated in the aqueous spaces between the biomolecular layers. Lipid soluble materials are incorporated into the lipid layers although polar head groups may protrude from the layer into the aqueous space. The encppsulation of these compounds can be achieved by a number of methods. The method most commonly used involves casting a thin film of phospholipid onto the wall of a flask by evaporation of an organic solvent. When this film is dispersed in a suitable aqueous medium, multilamellar liposomes are formed (also referred to as coarse liposomes). Upon suitable sonication, the coarse liposomes form smaller similarly closed vesicles.
Water-soluble biologically active compounds are usually incorporated by dispersing the cast film with an aqueous solution of the compound. The unencapsulated compound is then removed by centrifugation, chromatography, dialysation or some other suitable procedure. Lipid-soluble biologically active compounds are usually incorporated by dissolving them in the organic solvent with the phospholipid prior to casting the film. Providing the solubility of these compounds in the lipid phase is not exceeded or the amount present is not in excess of that which can be bound to the lipid, liposomes prepared by the above method usually contain most of the compound bound in the lipid bilayers; separation of the liposomes from unencapsulated material is not required. Other methods of preparing liposomes have been described although these are mainly specialized methods producing unilamellar liposomes and include reverse-phase evaporation of an organic solvent from a water-in-oil emulsion of phospholipid, infusion of organic solutions of phospholipid into large volumes of aqueous phase and detergent removal from mixed micelles of detergent and lipid.
Aqueous liposome dispersions only have limited physical stability. The liposomes can aggregate and precipitate as sediment. Although this sediment may be redispersed, the size distribution may be different from that of the original dispersion. This may be overcome to some extent by incorporation of caarged lipids into the liposomes. In addition, on storage the biologically active compounds may be lost into the external aqueous phase which restricts the potential of these preparations as practical dosage forms. This is particularly notable for low molecular weight water-soluble compounds but lipid soluble compounds too can partition into the external aqueous medium. If the volume of the aqueous medium is large, this loss can be significant. In addition, depending upon the type of lipid and biologically active compound present in the liposome, there is the potential for chemical degradation of the lipid components and/or the biologically active components in the aqueous dispersion.
These factors restrict the use of liposomes as practical carriers of biologically active compounds. One solution suggested for overcoming the limited physical stability of liposomes is to prepare and store the lipid/biologically active compound film and then disperse the film to form liposomes just prior to administration. However, unit dose film preparation presents serious practical difficulties in that the containers would require a high surface area to facilitate solvent evaporation and deposition of a thin film suitable for rapid rehydration to form liposomes readily. This type of container by virtue of its bulk would present severe storage problems. Other methods suggested for preparing liposome components in a solid form for storage have included freeze-drying the prepared aqueous liposome suspension as described in U.S. Pat. Nos. 4,229,360 and 4,247,411 and by freeze-drying the liposome components from a suitable organic solvent as described in U.S. Pat. No. 4,311,712. These freeze-dried preparations result in a porous matrix of liposome components which is easily hydrated.